Method for electrophoretically immersion-enameling substrates that have edges

ABSTRACT

Method of electro-dipcoating while reducing edge migration on stoving by 
     1) electro-deposition of a coating layer from an electrically depositable coating composition containing a heat-curable binder system having a content of olefinically unsaturated double bonds that are radically polymerisable under UV irradiation, on an electrically conductive substrate having edges, 
     2) UV irradiation of at least part of the electrically deposited coating layer, avoiding complete curing, 
     3) complete curing of the electrically deposited coating layer by stoving.

This application is 35 U.S.C. 371 National Stage filing ofPCT/EP00/03898 filing Apr. 29, 2000.

BACKGROUND OF THE INVENTION

The invention relates to a method of lacquering electrically conductivesubstrates with electrically depositable aqueous electro-dipcoatinglacquers (EDL). The invention relates also to a method of preventingedge migration on the stoving of electrically deposited EDL coatinglayers.

Electro-dipcoating lacquers are used especially to produceanti-corrosive primer coats on metallic substrates. However, they may,for example, also be deposited and stoved on any desired electricallyconductive substrates as a single-layer finishing lacquer, a clearlacquer, or as a lacquer layer that is arranged within a multilayerlacquer coating. An EDL coating layer arranged within a multilayerlacquer coating may be, for example, a lacquer layer having a decorativeeffect, which serves as a finishing lacquer or may be covered with aclear lacquer layer.

A problem in the case of lacquering with electro-dipcoating lacquers isedge migration on stoving of an EDL coating layer deposited beforehandon an electrically conductive substrate. The EDL coating layer pullsaway from the edge, with a reduction in the layer thickness at or in theimmediate vicinity of the edge. In the extreme case, the edge isinsufficiently covered after stoving. While this is noticeable, forexample, in the case of decorative EDL coatings as a difference incolour because the substrate shows through in the region of the edge, itresults in the case of anti-corrosive EDL primer coats in an impairmentor in the loss of the anti-corrosive action at or in the region of theedge. Apart from the technical disadvantages of edge corrosion,corrosion at edges that are accessible to an observer is troublesome inparticular from a visual point of view, for example in the form ofvisible rust spots and runs that form during use of the lacqueredsubstrates. EDL coating compositions that are curable by irradiationwith ultraviolet light, and the curing by UV irradiation of EDL coatinglayers deposited electrically from such EDL coating compositions, areknown inter alia from U.S. Pat. Nos. 4,040,925 and 4,039,414.

SUMMARY OF THE INVENTION

The object of the invention is to provide a method of electro-dipcoatingwith which EDL coatings with good edge coverage can be obtained onelectrically conductive substrates having edges, that is to say thedeposited EDL coating layer is to exhibit no or only slight edgemigration behaviour on stoving. In particular, edge migration at edgesthat are accessible to an observer is to be avoided, in order to avoidthe undesirable occurrence of visually disturbing corrosion phenomena atsuch edges during subsequent use of the lacquered substrates.

It has been found that that object can be achieved if a heat-curable EDLcoating composition containing a binder system having a content ofradically polymerisable olefinically unsaturated double bonds is usedfor the electro-dipcoating of an electrically conductive substratehaving edges, and if, before stoving of the coating layer depositedelectrically from the EDL coating composition, UV irradiation of theelectrically deposited coating layer is carried out at least at edges ofthe substrate and the wholly or partially UV-irradiated EDL coatinglayer is then cured by stoving. The term “curing” used here andhereinbelow means curing in the sense of a chemical crosslinking of theEDL coating layer by formation of covalent bonds between theconstituents of the heat-curable EDL binder system.

Only so-called dual-cure systems have hitherto been known from theliterature. In such systems, curing takes place in the opposite manner.For example, U.S. Pat. No. 4,066,523 and U.S. Pat. No. 4,070,258describe electro-dipcoating lacquers and combinations of polymers withtertiary amino groups, polymers with lateral mercapto groups, abismaleimide crosslinking agent or an ethylenically unsaturated carbonylcrosslinking agent, which are cured after deposition first by means ofheat and finally by UV irradiation.

Accordingly, the present invention provides a method ofelectro-dipcoating, consisting of the successive steps:

1) electro-deposition of a coating layer from an EDL coating compositioncontaining a heat-curable binder system having a content of olefinicallyunsaturated double bonds that are radically polymerisable under UVirradiation, on an electrically conductive substrate having edges,

2) UV irradiation of the electrically deposited coating layer, at leastin the region of the edges, and then

3) curing of the EDL coating layer by stoving.

A preferred embodiment of the invention comprises a method ofelectro-dipcoating consisting of the successive steps:

1) electro-deposition of a coating layer from an EDL coating compositioncontaining a heat-curable binder system having a content of olefinicallyunsaturated double bonds that are radically polymerisable under UVirradiation, on an electrically conductive substrate having edges,

2) UV irradiation of the electrically deposited coating layer in theregion of the edges, especially of the electrically deposited coatinglayer at the edges,

3) curing of the EDL coating layer by stoving.

The method according to the invention can advantageously be used for theelectro-dipcoating of three-dimensional, electrically conductive,especially metallic, substrates having edges and having regions that aredirectly accessible and regions that are not accessible to an observer,especially having edges that are directly accessible and edges that arenot accessible to an observer. “Directly accessible to an observer”means “accessible to the eye of an observer from the outside withoutparticular technical or optical aids”. Only regions or edges that aredirectly accessible to an observer are also directly accessible to UVirradiation. Examples of substrates having regions that are directlyaccessible and regions that are not accessible to an observer,especially having edges that are directly accessible and edges that arenot accessible to an observer, are especially motor vehicle bodies withtheir hollow spaces, joins and other construction-related undercuts.Examples of edges of motor vehicle bodies that are directly accessibleto an observer are externally visible cut edges of individual bodycomponents, the edges of holes, for example of clip holes or of openingsprovided for components that are to be inserted, such as windows, headlamps, door locks or door handles, and the edges, of roof rails.

DETAILED DESCRIPTION OF THE INVENTION

Accordingly, the invention relates in a particular embodiment to amethod of electro-dipcoating consisting of the successive steps:

1) electro-deposition of a coating layer from an EDL coating compositioncontaining a heat-curable binder system having a content of olefinicallyunsaturated double bonds that are radically polymerisable under UVirradiation, on a three-dimensional, electrically conductive substratehaving edges and having regions that are accessible and regions that arenot accessible to an observer,

2) UV irradiation of the electrically deposited coating layer in regionsof the substrate surface that are directly accessible to an observer,

3) curing of the EDL coating layer by stoving.

The particular embodiment of the method according to the invention ispreferably a method of electro-dipcoating consisting of the successivesteps:

1) electro-deposition of a coating layer from an EDL coating compositioncontaining a heat-curable binder system having a content of olefinicallyunsaturated double bonds that are radically polymerisable under UVirradiation, on a three-dimensional, electrically conductive substratehaving edges that are directly accessible and edges that are notaccessible to an observer,

2) UV irradiation of the electrically deposited coating layer in theregion of edges of the substrate that are accessible to an observer,especially the EDL-coated edges that are directly accessible to anobserver,

3) curing of the EDL coating layer by stoving.

The electro-dipcoating lacquers used in the method according to theinvention are aqueous coating compositions having, for example, from 10to 30 wt. % solids. They may be anodically or cathodically depositableelectro-dipcoating lacquers. The solids in the electro-dipcoatinglacquers used in the method according to the invention are formed by theresin solids of the EDL binder system, optionally present reactivediluents (compounds which are incorporated chemically into the lacquerfilm on UV irradiation and/or during the stoving process), pigments,fillers and further non-volatile additives conventionally employed inlacquers. Solids here and hereinbelow means theoretical solids; it doesnot take account of any losses, for example losses on evaporation and/orstoving during application, UV irradiation and curing of the EDL coatingcomposition. The binder system of the EDL coating compositions used inthe method according to the invention is composed of one or more EDLbinders, crosslinking agents that may optionally be present, pasteresins that may optionally be present and non-ionic additional resinsthat may optionally be present. For example, the EDL binder system iscomposed of solids proportions by weight, which add up to 100 wt. %, offrom 50 to 100 wt. % EDL binder, from 0 to 50 wt. % crosslinking agent,from 0 to 30 wt. % non-ionic additional resins and from 0 to 20 wt. %paste resin. The crosslinking agents, non-ionic additional resins and/orpaste resins may optionally be identical substances which simultaneouslyperform two or three functions in the EDL coating composition, forexample serve both as non-ionic additional resin and as crosslinkingagent or both as crosslinking agent and as paste resin. The sum of theby-weight solids of crosslinking agent, non-ionic additional resin andpaste resin is not more than 50 wt. % of the resin solids in the EDLbinder system. The binder systems contained in the EDL coatingcompositions used in the method according to the invention are bindersystems that are conventionally employed for electro-dipcoating lacquersand that are curable by means of heat, especially by stoving, andcontain olefinically unsaturated double bonds that are radicallypolymerisable under UV irradiation. The EDL binders and/or thecrosslinking agents preferably contain olefinically unsaturated doublebonds that are radically polymerisable under UV irradiation. Thenon-ionic additional resins and paste resins optionally contained in theEDL coating compositions may likewise contain olefinically unsaturateddouble bonds that are radically polymerisable under UV irradiation. Boththe paste resins and the non-ionic additional resins may be reactive ornon-reactive within the EDL binder system, that is to say they may beincluded or not included in the curing of step 3), regardless of whetherthey themselves contain olefinically unsaturated double bonds that areradically polymerisable under UV irradiation.

Examples of olefinically unsaturated double bonds that are radicallypolymerisable, for example, under UV irradiation and are contained inthe EDL binder systems are vinylic C═C double bonds, (meth)allylic C═Cdouble bonds and C═C double bonds bonded directly to carbonyl groups,especially (meth)acrylic double bonds. The content of olefinicallyunsaturated double bonds that are radically polymerisable under UVirradiation in the EDL binder systems is such that radicalpolymerisation of the olefinically unsaturated double bonds of the EDLbinder systems can take place under UV irradiation. Depending on thenature (reactivity) and amount of the radically polymerisableolefinically unsaturated double bonds contained in the EDL bindersystems, the EDL binder systems may be systems that are not curable, forexample not completely curable, by radical polymerisation under UVirradiation, are so curable only with difficulty or are readily socurable. For example, the EDL binder systems contain radicallypolymerisable olefinically unsaturated double bonds according to a C═Cequivalent weight of the resin solids of, for example, from 250 to10,000, preferably from 500 to 10,000. In the case of EDL binder systemsthat are heat-curable by means of radical polymerisation, the C═Cequivalent weight of the resin solids is in the lower range, for examplefrom 250 to 2000. Preference is given, however, to EDL binder systemsthat are heat-curable by addition and/or condensation reactions, and theC═C equivalent weight of the resin solids in that case is higher, forexample from 250 to 10,000, preferably from 500 to 3000. The C═Cequivalent weight of the resin solids of the EDL binder system denotesthe amount of resin solids, in grams, that contains one mole of olefinicdouble bonds. The radically polymerisable olefinically unsaturateddouble bonds may be part of the polymer backbone of polymericconstituents of the EDL binder system, especially of the EDL bindersand/or of the crosslinking agents, and/or may be present as lateraland/or terminal functional groups of polymeric constituents of the EDLbinder system, especially of the EDL binders and/or of the crosslinkingagents. The double bonds can be introduced into the polymericconstituents of the EDL binder system, especially of the EDL bindersand/or crosslinking agents, by various organo-chemical methods known tothe person skilled in the art That may be effected, for example, by theuse of corresponding low molecular weight olefinically unsaturatedcompounds, for example during and/or after conclusion of the actual EDLbinder and/or crosslinking agent synthesis in the practically finishedEDL binder and/or crosslinking agent by polymer-analogous reaction withcorresponding low molecular weight olefinically unsaturated compounds.Examples which may be mentioned are the addition of epoxy-functionalolefinically unsaturated compounds, such as, for example, glycidyl(meth)acrylate, to carboxyl- or hydrogen-active amino groups of the EDLbinder and/or crosslinking agent; the addition of isocyanate-functionalolefinically unsaturated compounds, such as, for example,isocyanatoalkyl (meth)acrylate, 3-isopropenylalpha,alpha-dimethylbenzylisocyanate, or isocyanate-functional adductsof polyisocyanate and hydroxy-functional olefinically unsaturatedcompounds such as hydroxyalkyl (meth)acrylate or (meth)allyl alcohol, tohydroxyl- and/or hydrogen-active amino groups of the EDL binder and/orcrosslinking agent; the addition of hydroxy-functional olefinicallyunsaturated compounds such as hydroxyalkyl (meth)acrylate or (meth)allylalcohol to isocyanate groups of the EDL binder and/or crosslinkingagent; and/or the addition of carboxy-functional olefinicallyunsaturated compounds such as (meth)acrylic acid to epoxy groups of theEDL binder and/or crosslinking agent.

The EDL coating compositions used in the method according to theinvention may be anodically or cathodically depositableelectro-dipcoating lacquers. The EDL binders therefore carry ionicsubstituents and/or substituents that can be converted into ionicgroups. The crosslinking agents optionally contained in the EDL coatingcompositions may also have ionic groups and/or groups that can beconverted into ionic groups. The ionic groups or groups that can beconverted into ionic groups may be anionic groups or groups that can beconverted into anionic groups, for example acid groups, such as —COOH,—SO₃H and/or —PO₃H₂ and the corresponding anionic groups neutralisedwith bases. However, the ionic groups may also be cationic groups orgroups that can be converted into cationic groups, for example basicgroups, preferably nitrogen-containing basic groups; such groups may bepresent in quaternary form, or they are converted into cationic groupswith a conventional neutralising agent, for example an organicmonocarboxylic acid, such as, for example, formic acid or acetic acid.Examples are amino, ammonium, for example quaternary ammonium,phosphonium and/or sulfonium groups. Amino groups that are present maybe primary, secondary and/or tertiary. The groups that can be convertedinto ionic groups may be present wholly or partially in neutralisedform.

The EDL coating compositions that can be used in the method according tothe invention may be anodically depositable EDL coating compositions(ADL) known per se. They contain anodically depositable binders, forexample based on polyesters, epoxy resin esters, (meth)acrylic copolymerresins, maleinate oils or polybutadiene oils, for example having aweight-average molar mass (Mw) of from 300 to 10,000 and, for example,an acid number of from 35 to 300 mg KOH/g. The binders carry, forexample, COOH, SO₃H and/or PO₃H₂ groups. The resins may be convertedinto the aqueous phase after neutralisation of at least some of the acidgroups.

EDL coating compositions used in the method according to the inventionespecially as an anti-corrosive primer coat are preferably cathodicallydepositable EDL coating compositions (CDL) known per se. They containcathodically depositable binders, for example resins containing primary,secondary and/or tertiary amino groups, the amine numbers of whichresins are, for example, from 20 to 250 mg KOH/g. The weight-averagemolar mass (Mw) of such CDL binders is preferably from 300 to 10,000.The resins can be converted into the aqueous phase after quaternisationor neutralisation of at least some of the basic groups. Examples of suchCDL binders are aminoepoxy resins, amino(meth)acrylate resins,aminopolyurethane resins, polybutadiene resins containing amino groups,and/or modified epoxy resin-carbon dioxide-amine reaction products.

The EDL binders can be self-crosslinking or crosslink by external means;in the latter case, they carry groups capable of chemical crosslinkingand the EDL coating compositions then contain crosslinking agents. TheEDL binder systems are curable by means of heat, especially by stoving.Heat-curing may be curing of the EDL binder system by radicalpolymerisation of olefinically unsaturated double bonds and/or bycondensation reactions and/or addition reactions. The EDL binder systemsmay be in the form of mixtures of EDL binder systems that areheat-curable by radical polymerisation of olefinically unsaturateddouble bonds, and EDL binder systems that are curable by condensationreactions and/or addition reactions, and/or the EDL binder systemcontains one or more EDL binders that are heat-curable both by radicalpolymerisation of olefinically unsaturated double bonds and bycondensation reactions and/or addition reactions. Preference is given toEDL binder systems that are heat-curable by radical polymerisation ofolefinically unsaturated double bonds or that are heat-curable bycondensation reactions and/or addition reactions. Special preference isgiven to EDL binder systems that are heat curable by condensationreactions and/or addition reactions.

In the case of EDL binder systems that are heat-curable by radicalpolymerisation, the EDL binders contain radically polymerisableolefinically unsaturated double bonds according to a C═C equivalentweight of the resin solids of, for example, from 250 to 2000. The EDLbinders that are heat-curable by radical polymerisation may be present,for example, in combination with non-ionic radically polymerisableprepolymers (as representatives of non-ionic additional resins) and/orradically polymerisable reactive diluents (radically polymerisablemonomers).

Examples of non-ionic, radically polymerisable prepolymers or oligomers,which may be contained as non-ionic additional resins especially in EDLcoating compositions that are heat-curable by radical polymerisation,are (meth)acryl-functional (meth)acrylic copolymers, epoxy resin(meth)acrylates, polyester (methacrylates, polyether (meth)acrylates,polyurethane (meth)acrylates, unsaturated polyesters, unsaturatedpolyurethanes or silicone (meth)acrylates having number-averagemolecular masses (Mn) preferably in the range from 200 to 10,000,particularly preferably from 500 to 3000, and having on average from 2to 20, preferably from 3 to 10, radically polymerisable olefinic doublebonds per molecule.

The radically polymerisable reactive diluents which may be contained inthe EDL coating compositions in amounts of from 0 to 20 wt. %, based onthe resin solids of the EDL binder system, are defined low molecularweight compounds which may be mono-, di- or poly-unsaturated. Examplesof such reactive diluents are: (meth)acrylic acid esters, vinyl acetate,vinyl ethers, substituted vinylureas, ethylene and propylene glycoldi(meth)acrylate, 1,3- and 1,4-butanediol di(meth)acrylate, vinyl(meth)acrylate, allyl (meth)acrylate, glycerol tri-, di- andmono(meth)acrylate, trimethylolpropane tri-, di- andmono-(meth)acrylate, styrene, vinyltoluene, divinylbenzene,pentaerythritol tri- and tetra-(meth)acrylate, di- and tri-propyleneglycol di(meth)acrylate, hexanediol di(meth)acrylate.

In the case of EDL binder systems that are heat-curable by condensationand/or addition reactions, the EDL binders contain one or morefunctional groups that are amenable to thermally induced chemicalcrosslinking by condensation and/or addition reactions. If the EDLbinders are self-crosslinking, they possess reactive groups that arecomplementary to one another as the base for a thermally inducedcovalent crosslinking. In the case of EDL binders that crosslink byexternal means, which are preferred, the choice of crosslinking agentspresent, which in itself is not critical, depends on the functionalityof the EDL binders, that is to say the crosslinking agents are so chosenthat they have a reactive functionality complementary to thefunctionality of the EDL binders, it being possible for the functionalgroups to react with one another thermally with addition and/orcondensation. Examples of addition reactions suitable for thecrosslinking of the EDL binder systems are the ring-opening addition ofan epoxy group to a carboxyl-, hydroxyl- or hydrogen-active amino group,the ring-opening addition of a cyclic carbonate group to ahydrogen-active amino group, or the addition of a C═C double bond bondeddirectly to a carbonyl group, especially a (meth)acrylic double bond, toa hydroxyl- or hydrogen-active amino group. Examples of condensationreactions suitable for the crosslinking of the EDL binder systems arethe reaction of a hydroxyl- or hydrogen-active amino group with ablocked isocyanate group with formation of a urethane or urea group andremoval of the blocking agent, the reaction of a hydroxyl group with anN-methylol group with removal of water, the reaction of a hydroxyl groupwith an N-methylol ether group with removal of the etherifying alcohol,the transesterification reaction of a hydroxyl group with an ester groupwith removal of the esterifying alcohol, the transamidation reaction ofa hydrogen-active amino group with an ester group with removal of theesterifying alcohol. It is also possible for a plurality ofcomplementary functionalities to be present side by side in an EDLbinder or EDL binder system that is heat-curable by addition and/orcondensation reactions, provided such functionalities are compatiblewith one another, so that two or more different reactions of the typementioned above by way of example may occur during stoving. Thecrosslinking agents optionally used in the EDL binder systems may bepresent individually or in a mixture. Some examples of crosslinkingagents suitable for use in EDL binder systems that crosslink by externalmeans are listed below:

1) Crosslinking agents that are used preferably in combination with EDLbinders having carboxyl-, hydroxyl- or hydrogen-active amino groups andthat have epoxy groups in the molecule: polyepoxides having epoxy groupsbonded directly to an alicyclic or bridged alicyclic ring, polyglycidylcompounds, such as polyglycidyl ethers, for example aromatic epoxyresins based on bisphenol A, polyglycidyl esters, epoxy-functionalnovolaks, epoxy-functional copolymers, for example copolymers ofglycidyl (meth)acrylate, epoxidised polybutadiene, or polyepoxycompounds formed by targeted synthesis, for example addition products ofepoxy-functional alcohols, such as, for example,3,4-epoxytetrahydrobenzyl alcohol, with polyisocyanates, for examplepolyisocyanates conventionally employed in lacquers, polyurethaneprepolymers having free NCO groups, or (meth)acrylic copolymers.

2) Crosslinking agents that are used preferably in combination with EDLbinders having hydrogen-active amino groups and that have cycliccarbonate groups in the molecule: compounds having 5- or 6-memberedcyclic carbonate groups, preferably having 2-oxo-1,3-dioxolan-4′-ylgroups, for example prepared by reaction of carbon dioxide with theoxirane rings of polyepoxide or polyglycidyl compounds listed aboveunder 1) or synthesised in a targeted manner with the use of suitablemonomer compounds containing a cyclic carbonate group, for example byaddition of hydroxy-functional cyclocarbonates, such as, for example,4-hydroxymethyl-2-oxo-1,3-dioxolan, to polyisocyanates, for examplepolyisocyanates conventionally employed in lacquers, polyurethaneprepolymers having free NCO groups, or (meth)acrylic copolymers.

3) Crosslinking agents that are used preferably in combination with EDLbinders having hydroxyl- and/or hydrogen-active amino groups and thathave blocked isocyanate groups in the molecule: blocked polyisocyanatesconventionally employed in lacquers. Examples thereof are any desireddi- and/or polyisocyanates in which the isocyanate groups have beenreacted with a blocking agent (a monofunctional compound containingactive hydrogen). Examples of polyisocyanates are aromatic, araliphaticand (cyclo)aliphatic diisocyanates, such as, for example, hexamethylenediisocyanate, (methyl)cyclohexane diisocyanate, tetramethylxylylenediisocyanate, isophorone diisocyanate, biscyclohexylmethanediisocyanate, toluylene diisocyanate, diphenylmethane diisocyanate, aswell as oligomers derived from diisocyanates. Examples of such oligomersare polyisocyanates formed by dimerisation or trimerisation, as well asreaction products of stoichiometrically excess diisocyanate with water,amines or polyols. Such polyisocyanates contain uretdione, isocyanurate,biuret, allophanate, urea and/or urethane groups. Examples of blockingagents are alcohols, such as n-butanol, isopropanol, 2-ethylhexanol,(meth)allyl alcohol, hydroxyalkyl (meth)acrylates, for examplehydroxyethyl (meth)acrylate; phenols; oximes such as methyl ethylketoxime, acetone oxime; lactams such as epsilon-caprolactam; imidazoleor pyrazole derivatives; CH-acidic compounds such as beta-diketones, forexample acetylacetone, malonic acid dialkyl esters or acetic acid alkylesters.

4) Crosslinking agents that are used preferably in combination with EDLbinders having hydroxyl groups and that have methylol and/or methylolether groups in the molecule: preferably aminoplastic resinsconventionally employed in lacquers, especially triazine resins such asmelamine resins or benzoguanamine resins.

5) Crosslinking agents that are used preferably in combination with EDLbinders having hydroxyl- and/or hydrogen-active amino groups and thathave in the molecule ester groups capable of transesterification ortransamidation: polyesters having terminal end groups of the —COOalkyltype, especially beta-hydroxy ester end groups,tris(alkoxycarbonylamino)-1,3,5-triazines (TACT).

6) Crosslinking agents that are used preferably in combination with EDLbinders having hydroxyl- and/or hydrogen-active amino groups and thathave C═C double bonds bonded directly to carbonyl groups, especially(meth)acrylic double bonds. Examples are inter alia the non-ionic,radically polymerisable prepolymers or oligomers mentioned above, aswell as adducts of polyisocyanates mentioned under 3) and hydroxyalkyl(meth)acrylates.

For the production of the EDL coating compositions used in the methodaccording to the invention, the EDL binders may be used in the form ofan aqueous EDL binder dispersion, which may optionally contain, forexample, crosslinking agents. EDL binder dispersions can be prepared bysynthesis of EDL binders in the presence or absence of organic solventsand conversion into an aqueous dispersion by dilution with water of theEDL binders neutralised with neutralising agents. The EDL binders can beconverted into the aqueous dispersion in admixture with, for example,crosslinking agents. Organic solvent, where present, can be removed tothe desired content, for example by distillation in vacuo, before orafter conversion into the aqueous dispersion.

In addition to the EDL binders, water and any crosslinking agents,non-ionic additional resins, unsaturated prepolymers, reactive diluentsand/or paste resins that may be present, the EDL coating compositionsused in the method according to the invention may contain pigments,fillers, photoinitiators, heat-activatable radical initiators, solvents,and/or additives conventionally employed in lacquers.

Examples of pigments are the conventional inorganic and/or organiccoloured pigments and/or effect pigments, such as, for example, titaniumdioxide, iron oxide pigments, carbon black, phthalocyanine pigments,quinacridone pigments, metal pigments, for example of titanium,aluminium or copper, interference pigments, such as, for example,titanium-dioxide-coated aluminium, coated mica. Examples of fillers arekaolin, talcum or silicon dioxide. The nature and amount of the pigmentsis dependent on the intended use of the EDL coating compositions.

The pigments and/or fillers can be dispersed in a portion of the EDLbinder and then milled in a suitable apparatus, for example a bead mill,following which completion is effected by mixing with the remainingportion of EDL binder. It is then possible to prepare the EDL coatingcomposition or bath from that material—after the addition ofneutralising agents, where not already added—by dilution with water(single-component method).

Pigmented EDL coating compositions or baths can, however, also beprepared by mixing an EDL binder dispersion and a pigment paste that hasbeen prepared separately (two-component method). To that end, an EDLbinder dispersion is further diluted, for example, with water, and anaqueous pigment paste is then added. Aqueous pigment pastes are preparedby methods known to the person skilled in the art, preferably bydispersion of the pigments and/or fillers in paste resins conventionallyemployed for such purposes.

The weight ratio of pigment plus filler/binder plus crosslinking agentin the EDL coating compositions used in the method according to theinvention is, for example, from 0:1 to 0.8:1; for pigmented lacquers itis preferably from 0.05:1 to 0.4:1.

The EDL coating compositions used in the method according to theinvention may contain volatile and/or non-volatile additives, forexample in amounts of from 0.1 to 5 wt. %, based on the resin solids.Such additives are especially those which are known for EDL coatingcompositions, for example wetting agents, neutralising agents, flowagents, catalysts, corrosion inhibitors, antifoams, light stabilisers,antioxidants, colourings, biocides and conventional anti-pittingadditives.

The EDL coating compositions used in the method according to theinvention may contain photoinitiators, for example in amounts of from0.1 to 5 wt. %, based on the resin solids. It is advantageous if theirabsorption is in the wavelength range from 260 to 450 nm. Examples ofphotoinitiators, which may be contained in the EDL coating compositionsalone or in a mixture, are benzoin and its derivatives, acetophenone andits derivatives, for example 2,2-diacetoxyacetophenone, benzophenone andits derivatives, thioxanthone and its derivatives, anthraquinone,1-benzoylcyclohexanol, organophosphorus compounds, such as, for example,acylphosphine oxides.

The EDL coating compositions used in the method according to theinvention, especially the EDL coating compositions that are heat-curableby radical polymerisation, may contain heat-activatable radicalinitiators. Examples of thermolabile radical initiators are organicperoxides, organic azo compounds or C—C-cleaving initiators, such asdialkyl peroxides, peroxocarboxylic acids, peroxodicarbonates, peroxideesters, hydroperoxides, ketone peroxides, azodinitriles or benzpinacolsilyl ether. The preferred amounts used are from 0.1 to 5 wt. %, basedon the resin solids.

The additives, photoinitiators and thermolabile radical initiators canbe introduced into the EDL coating compositions in any desired manner,for example during synthesis of the binders, during the preparation ofEDL binder dispersions, via a pigment paste or, alternatively,separately.

The EDL coating compositions used in the method according to theinvention may also contain conventional solvents in the amountsconventionally employed for EDL coating compositions. Examples areglycol ethers, such as butyl glycol and ethoxypropanol, and alcohols,such as butanol. The solvents can be introduced into the EDL coatingcompositions in various ways, for example as a constituent of binder orcrosslinking agent solutions, via an EDL binder dispersion, as aconstituent of a pigment paste or, alternatively, by separate addition.The solvent content of the EDL coating compositions is, for example,from 0 to 5 wt. %, based on coatable EDL bath.

The EDL coating compositions used in the method according to theinvention can be prepared by the known methods for the preparation ofEDL baths, that is to say in principle either by means of theabove-described single-component method or by means of the two-componentmethod.

In the context of the method according to the invention, the EDL coatingcompositions used in the method according to the invention can beapplied in the conventional manner, by electro-deposition, in thecontext of a single-layer or multi-layer lacquering, to varioussubstrates that have edges and that are electrically conductive or havebeen rendered electrically conductive, for example that have beenprovided with an electrically conductive coating layer, especiallymetallic substrates. As has been mentioned above, the edges can beaccessible to an observer, or to the UV irradiation carried out in step2), either completely or only in part.

The method according to the invention is suitable especially for themotor vehicle sector, for example for the application of anticorrosiveEDL primer coats to motor vehicle bodies or parts of motor vehiclebodies. The EDL primer coats may optionally be provided with furtherlacquer layers. However, it is also possible in the method according tothe invention to deposit the EDL coating compositionselectrophoretically as, for example, a finishing lacquer, a clearlacquer, or as a lacquer layer that is arranged within a multilayerlacquer coating and may have a decorative function.

In addition to the above-mentioned content in the EDL binder systems ofolefinically unsaturated double bonds that are radically polymerisableunder UV irradiation, it is essential to the invention that the coatinglayers deposited electrophoretically in the conventional manner from theEDL coating compositions on substrates having edges be subjected to UVirradiation before the stoving operation effecting curing of the EDLcoating layers. Depending on the embodiment of the method according tothe invention, the entire surface of the uncured EDL coating layer or aportion thereof, for example only one or more edges of the substrateprovided with the uncured EDL coating layer, is irradiated with UV. TheUV irradiation of the uncured EDL coating layer carried out in step 2)of the method according to the invention leads to radical polymerisationof olefinically unsaturated double bonds in the EDL binder system in theuncured EDL coating layer, but does not lead to curing of the EDLcoating layer. Curing of the EDL coating layer is effected only in step3) of the method according to the invention by stoving. For example,after the UV irradiation the EDL coating layer does not achieve thependulum hardness that will be reached after the subsequent heat-curingby stoving and/or can, for example, still be removed by wiping severaltimes with a cotton wool swab soaked with solvent. After stoving, theEDL coating layer is resistant to solvents and cannot be removed even bywiping more tan 100 times with a cotton wool swab soaked with solvent.Incomplete curing of the EDL coating layer during step 2) of the methodaccording to the invention can be ensured, for example, by suitableselection of the composition of the EDL coating composition and/or themanner in which the method is carried out in step 2). For example, thepigmenting, the nature and amount of the photoinitiators and the EDLbinder system in the EDL coating composition can be so chosen thatcuring of the EDL coating layer deposited from the EDL coatingcomposition is impossible or is possible only with difficulty in step 2)of the method according to the invention. For example, a pigmenting thatmore or less absorbs UV radiation can be chosen, and/or no or only asmall amount of photoinitiator is used, and/or the EDL binder system iscurable only with difficulty or is not curable by radicalpolymerisation. The parameters of step 2) can also be influenced in anappropriate manner, as will be discussed further below.

Following the EDL coating of step 1), the EDL coating layer is exposedto UV radiation. Suitable UV sources are, for example, those havingemissions in the wavelength range from 180 to 420 nm, preferably from200 to 400 nm. Examples of UV sources are optionally dopedhigh-pressure, medium-pressure and low-pressure mercury emitters,discharge tubes, such as, for example, low-pressure xenon lamps, UVpoint emitters, black-light tubes, high-energy electronic flash devices,such as, for example, UV flash lamps.

The UV sources may be designed to operate continuously ordiscontinuously. A possibility for UV sources that can be switched onand off for a short time (timed) consists in the upstream provision of,for example, movable diaphragms, or UV flash lamps are used.

The arrangement of the UV sources is known in principle; it can beadapted to the conditions of the substrate, for example of a motorvehicle body, or the edges of the substrate that are to be irradiated.For example, the substrate can be irradiated as a whole, for example asit passes through a UV irradiation tunnel, or it is possible to use aradiation curtain, which moves relative to the substrate. In addition, apoint-like UV source or an emitter of small area can be guided over thesubstrate by means of an automatic device. In a corresponding manner itis possible to expose to UV radiation only the edges of the substrate orregions of the substrate having edges.

The distance of the UV source can be fixed, or it is adapted to thesubstrate at a desired value, for example to the shape of the substrateor the arrangement of the edges of the substrate. The distances of theUV sources are, for example, in the range from 2 to 50 cm relative tothe surface of the EDL coating layer.

The irradiation time is, for example, in the range of the duration of aUV flash of, for example, from 100 milliseconds to 5 minutes, dependingon the irradiation method used and the nature and number of the UVsources. Preference is given to an irradiation time, that is to say thetime during which the UV radiation actually acts upon the uncured EDLcoating layer, of less than 5 minutes.

The energy supplied to the uncured EDL coating layer by UV irradiationduring step 2) of the method according to the invention is notsufficient to cure it. If the composition of the EDL coating compositionused in the method according to the invention is such that the EDLcoating layer is completely curable by UV irradiation in the course ofthe radical polymerisation, then the UV irradiation is so carried outthat complete curing of the EDL coating layer is definitely avoided. Themeasures suitable therefor are known to the person skilled in the art;for example, the duration of action of the UV radiation, the distance ofthe UV source from the EDL coating layer, the wavelength and/or outputof the UV source can be chosen accordingly.

The UV irradiation of step 2) is carried out before the stoving of step3). The UV irradiation and the stoving can be separate from one anotherin terms of space and time. For example, the UV sources may be locatedoutside the stoving oven. In the case of goods produced in series, it isalso possible for the UV sources to be located at the beginning of thestoving oven or in the front region thereof, for example in the frontthird of the stoving oven. For example, the UV sources may be arrangedin the inlet region of the stoving oven. The method according to theinvention is then distinguished by the fact that the UV irradiation andthe stoving are partly carried out in parallel in terms of space andtime, but the UV irradiation of step 2) is already complete while thecuring of step 3) has not or has only just begun, for example while thesubstrate is still being heated.

After the UV irradiation, the still uncured EDL coating layer, which hasbeen subjected to UV radiation over its entire surface or over a portionthereof, is cured in step 3) by stoving. Depending on the nature of theEDL binder system, stoving is carried out, for example, for a durationof from 20 to 30 minutes at oven temperatures of from 80 to 220° C.

The method according to the invention permits the production of curedEDL coatings with good edge coverage on electrically conductivesubstrates having edges. The corrosion protection of EDL-primed edges,especially of metal substrates, which have been exposed to UV radiationbefore curing, is improved. The undesired occurrence of visuallydisturbing edge corrosion phenomena during subsequent use of thelacquered substrates can be avoided.

EXAMPLE 1 Preparation of a CDL Binder Solution

According to EP-B-12 463, 301 g of diethanolamine, 189 g of3-(N,N-dimethylamino)-propylamine and 1147 g of an adduct of 2 mol. of1,6-hexanediamine and 4 mol. of the glycidyl ester of versatic acid(Cardura® E 10 from Shell) are added to 5273 g of bisphenol A epoxyresin (epoxy equivalent weight 475) in 3000 g of ethoxypropanol. Thereaction mixture is maintained at from 85 to 90° C. for 4 hours, withstirring, and then at 120° C. for one hour. It is then diluted withethoxypropanol to 66% solids.

EXAMPLE 2 Preparation of a CDL Binder Solution

3120 g of binder solution are prepared according to DE-B-27 32 902,column 9, Example A2, from 706 g of bisphenol A epoxy resin (epoxyequivalent weight 260), 631 g of ethyl glycol acetate, 0.25 g ofhydroquinone, 765 g of semi-ester of tetrahydrophthalic anhydride andhydroxyethyl methacrylate, and 1017 g of a 70% solution of amonoisocyanate of toluylene diisocyanate and dimethylethanolamine inethyl glycol acetate, and the solution is mixed with 1930 g of a bindersolution from Example 1 diluted to 60% solids with ethoxypropanol. Thesolids content of the solution is 66%. The calculated double bondequivalent weight is 618, based on solid resin.

EXAMPLE 3 Preparation of a Crosslinking Agent Solution

875 g of the solution of an adduct of 2,4-toluylene diisocyanate andtrimethylolpropane (molar ratio 3:1), 75% in ethyl acetate, is dilutedto a solids content of 50% with xylene, and 0.25 g of hydroquinone isadded thereto. After addition of 348 g of hydroxyethyl acrylate, thereaction mixture is heated for approximately 8 hours, with refluxcooling, until the NCO value has fallen virtually to zero. Ethyl acetateis then removed by fractional distillation at temperatures below 100°C., optionally with the application of a vacuum, until a solids contentof 75% has been achieved. The mixture is then adjusted to a solidscontent of 70% with methyl isobutyl ketone. The calculated double bondequivalent weight is 335, based on solid resin.

EXAMPLE 4 Preparation of a Crosslinking Agent Solution

Example 3 is repeated, with the difference that 360 g of butyl glycolare used instead of 348 g of hydroxyethyl acrylate.

EXAMPLE 5 Preparation of a CDL Bath

505 g of the binder solution from Example 2 are mixed, with thoroughstirring, with 50 g of ethoxypropanol, 2.4 g of carbon black and 235 gof titanium dioxide, and the mixture is milled in a bead mill. The batchis completed with 273 g of the binder solution from Example 2, 161 g ofthe crosslinking agent solution from Example 3, 50 g of phenoxypropanoland 31 g of 50% aqueous formic acid. Dibutyltin dilaurate is mixed inhomogeneously corresponding to an amount of 0.5% tin, based on the resinsolids. A CDL bath is prepared using 4067 g of deionised water.

EXAMPLE 6 Preparation of a CDL Bath

The procedure of Example 5 is followed, with the difference that 161 gof the crosslinking agent solution from Example 4 are used instead of161 g of the crosslinking agent solution from Example 3.

EXAMPLE 7 Preparation of a CDL Bath

505 g of the binder solution from Example 1 are mixed, with thoroughstirring, with 20 g of ethoxypropanol, 2.4 g of carbon black and 235 gof titanium dioxide, and the mixture is milled in a bead mill. The batchis completed with 273 g of the binder solution from Example 1, 132 g ofthe crosslinking agent solution from Example 3, 50 g of phenoxypropanoland 31 g of 50% aqueous formic acid. Dibutyltin dilaurate is mixed inhomogeneously corresponding to an amount of 0.5% tin, based on the resinsolids. A CDL bath is prepared using 4067 g of deionised water.

EXAMPLE 8 Preparation of a CDL Bath

Example 7 is repeated, with the difference that 132 g of thecrosslinking agent solution from Example 4 are used instead of 132 g ofthe crosslinking agent solution from Example 3.

The CDL baths from Examples 5 to 8 are stirred, uncovered, for threedays, without the admission of light. Lacquer films are then depositedcathodically from each of the CDL baths on perforated (hole diameter 10mm), degreased, non-phosphated motor vehicle body sheets in a dry layerthickness of 20 μm, and the sheets are rinsed with deionised water.After an evaporation time of 30 minutes at room temperature, the testsheets are exposed to UV radiation and then stoved for 17 minutes at anobject temperature of 175° C. or are stoved under the same conditionswithout being exposed to UV radiation. The stoved test sheets areexposed to a salt spray according to DIN 50 021-SS for 120 hours. Afterthe exposure, the edges of the holes are evaluated for edge rust(characteristic values CV 0 to 5; CV 0, edges without rust; CV 1,occasional rust spots at edges; CV 2, rust spots at less than ⅓ of theedges; CV 3, ⅓ to ½ of the edges covered with rust; CV 4, more than ½ ofthe edges covered with rust; CV 5, edges completely rust). The resultsare to be found in the following Table:

CDL.bath without UV with UV¹⁾ with UV²⁾ 5 CV 5 CV 1 acc. inv. CV 0 acc.inv. 6 CV 5 CV 2-3 acc. inv. CV 2 acc. inv. 7 CV 5 CV 2 acc. inv. CV 1-2acc. inv. 8 CV 5 CV 5 CV 5 ¹⁾UV irradiation in a type U 300 - M- 2 TRIST belt device, medium-pressure mercury radiator, 100 W/cm, belt speed2 m/min, object distance 10 cm ²⁾As ¹⁾, but belt speed 1 m/min, objectdistance 10 cm acc. inv. = method according to the invention

What is claimed is:
 1. A method of electro-dipcoating comprising thesteps of: 1) electro-deposition of a coating layer from an electricallydepositable coating composition containing a heat-curable binder systemcontaining olefinically unsaturated double bonds that are radicallypolymerizable under UV irradiation, on an electrically conductivesubstrate having edges, 2) UV irradiation of at least part of theelectrically deposited coating layer to radically polymerize theolefinically unsaturated double bonds, while avoiding complete curing ofthe electrically deposited coating layer, 3) complete curing of theelectrically deposited coating layer by stoving.
 2. The method accordingto claim 1, wherein the electrically depositable coating compositionsare selected from the group consisting of anodic depositableelectro-dipcoating lacquers and cathodic electro-dipcoating lacquers. 3.The method according to claim 2, wherein the electro-dipping lacquersand aqueous coating compositions comprising about 10% to about 30%solids.
 4. The method according to claim 1, wherein the substrate is athree dimensional, electrically conductive substrate having edges andhaving regions that are accessible and having regions that are notaccessible to an observer.
 5. The method according to claim 4, whereinthe three dimensional substrate is at least a part of a motor vehiclebody.
 6. The method according claim 1, wherein the heat-curable bindersystem that contains radically polymerizable olefinically unsaturateddouble bonds is selected from the group consisting of vinylic C═C doublebonds, (meth)allylic C═C double bonds, and C═C double bonds bondeddirectly to carbonyl groups.
 7. The method according to claim 6, whereinthe heat-curable binder system contains radically polymerizableolefinically unsaturated double bonds according to a C═C equivalentweight of the resin solids of from 250 to 10,000.
 8. The methodaccording to claim 7, wherein the heat-curable binder system containsradically polymerizable olefinically unsaturated double bonds that are aconstituent of the binders and/or of the crosslinking agents.
 9. Themethod according to claim 1, wherein the heat-curable binder systemfurther comprises at least one electro-dipcoating binder, crosslinkingagents, paste resins, non-ionic resins, and combinations thereof. 10.The method according to claim 9, wherein the non-ionic additional resinsthat are heat curable by radical polymerization are selected from thegroup consisting of (meth)acryl-functional (meth)acrylic copolymers,epoxy resin (meth)acrylates, polyester (meth)acrylates, polyether(meth)acrylates, polyurethane (meth)acrylates, unsaturated polyesters,unsaturated polyurethanes, and silicone (meth)acrylates having a numberaverage molecular mass in the range of about 200 to about 10,000radically polymerisable olefinic double bonds per molecule.
 11. Themethod according to claim 1, wherein the heat-curable binder system arecurable by condensation reactions.
 12. The method according to claim 1,wherein the heat-curable binder systems are curable by additionreaction.
 13. The method according to claim 1, wherein theelectro-dipcoating compositions further comprise at least one pigment,filler, photoinitiator, heat-activatable radical initiator, solvent, andcombinations thereof.
 14. The method according to claim 13, wherein thepigment is selected from the group consisting of organic coloredpigment, inorganic colored pigment, effect pigment and combinationsthereof.
 15. The method according to claim 13, wherein the filler isselected from the group consisting of kaolin, talcum, and silicondioxide.
 16. The method according to claim 13, wherein thephotoinitiator is selected from the group consisting of benzion,benzoine derivatives, acetophenone, acetophenone derivatives,benzophone, benzophone derivatives, thioxanthone, thioxanthonederivatives, anthraquinone, 1-benzoylcyclohexanol, organophosphoruscompounds and combinations thereof.
 17. The method according to claim13, wherein the heat-activatable radical initiators selected from thegroup consisting of organic peroxides, organic azo compounds, and C—Ccleaving initiators.
 18. The method according to claim 13, wherein thesolvent is selected from the group consisting of glycol ethers andalcohols.
 19. The method according to claim 1, wherein the UVirradiation of the electrically deposited coating layer of step 2) iscarried out in the region of the edges.
 20. The method according toclaim 1, wherein the UV irradiation of the electrically depositedcoating layer of step 2) is carried out in the regions of the substratesurface visible to an observer.
 21. The method according to claim 1,wherein the UV irradiation of the electrically deposited coating layerof step 2) is carried out in the regions of the electro-dipcoated edgesof the substrate are directly accessible to an observer.
 22. The methodaccording to claim 1, wherein the UV irradiation of the electricallydeposited coating layer occurs at a wavelength in the range of about 180nm to about 420 nm.
 23. The method according to claim 1, wherein the UVirradiation of the electrically deposited coating layer operatescontinuously.
 24. The method according to claim 1, wherein the UVirradiation of the electrically deposited coating layer operatesdiscontinuously.
 25. A substrate obtained by electro-dipcoatingaccording to claim 1.